Attention has been given to a so-called self-luminous display device in which a pixel is formed of a light-emitting element such as a light-emitting diode (LED). As the light-emitting element used in such a self-luminous display device, an organic light-emitting diode (OLED) (also referred to as an organic EL element, an electroluminescence (EL) element) has attracted attention, and is used for an organic EL display, and the like.
Since the EL element is a self-luminous light-emitting element which has an electroluminescent layer between a pair of electrodes and which emits light by applying current between the electrodes, it has an advantage that visibility of a pixel is higher than that of a liquid crystal display, backlight is not needed, and response speed is high. Luminance of the light-emitting element is controlled by a current value flowing to the element.
The light-emitting element has properties in which a resistance value (internal resistance value) is changed by an environmental temperature (ambient temperature). In particular, when a room temperature is to be a normal temperature, the resistance value is decreased as the temperature becomes higher than the normal temperature, and the resistance value is increased as the temperature becomes lower than the normal temperature. Therefore, in a constant voltage drive, when the temperature becomes high, a current value is increased and higher luminance than desired luminance is obtained, and when the temperature becomes low, a current value is decreased and lower luminance than desired luminance is obtained. The light-emitting element which has been used in these days has properties in which a current value is decreased with time even if a predetermined voltage is applied.
Due to the properties of the light-emitting element mentioned above, variation in luminance is generated when the ambient temperature is changed and temporal change is occurred. In order to solve the problem of luminance variation in a light-emitting element due to the ambient temperature change and temporal change, it is proposed to provide a monitor element (for example, refer to patent document 1). One of the electrodes of the monitor element is connected with a constant-current source and an input of an amplifier (an input terminal), and an output of the amplifier (an output terminal) is connected with one of the electrodes of a light-emitting element provided for a pixel in a pixel portion. According to the structure, the current flowing through the light-emitting element of the pixel is kept constant based on temperature properties of the monitor light-emitting element. In this specification, “connected” means not only a direct connection but also an electrical connection. Thus, other elements and wirings may be formed between objects to be connected. In this specification, “overlap” means not only the case when elements constituting a display device are directly overlapped with each other, but also the case when the elements are overlapped with other elements interposed therebetween.
Patent document 1—Japanese Patent Laid-Open No. 2002-333861
According to the above structure, current flowing into the light-emitting element of the pixel can be kept constant even if the temperature of the light-emitting element (electroluminescent layer) is changed. Accordingly, the power consumption of the display device can be prevented from increasing even if the ambient temperature of the display device is increased, and luminance can also be kept constant.
Since the monitor element is not used in image display, a region provided with the monitor element (monitor element portion) is required to prevent light-transmittance of light generated from the monitor element. As a method for solving light leakage, there is a method to provide a light shielding film. Further, there is a method to provide projections on a reflecting surface of a cathode (a surface being in contact with the side of a light-emitting layer) of the monitor element and to scatter reflected light at the reflecting surface of the cathode.
The example of a structure of a light shielding film provided in a monitor element portion is described with reference to FIGS. 1 and 2. FIG. 1 is a layout of the monitor element portion, and FIG. 2 is a diagram showing a cross sectional structure when taken along a chained line A-A′ in FIG. 1. Although the same monitor element portion as FIG. 1A is shown in FIG 1B, a first electrode 207 is omitted in a region shown as a reference number 212, and a current supply line 202, a light shielding film 214, and a drain electrode 215 are omitted in a region shown as a reference number 213, so as to give a simplified explanation on the location of the TFT for driving the monitor element and the like.
As shown in FIG. 1 and FIG. 2, in the monitor element portion, each of the region surrounded by a control line 201 which provides electric potential to a gate wiring of a TFT 211 for driving a monitor element and a gate wiring 206, is provided with a monitor element including a first electrode (an anode or a cathode), an electroluminescent layer 208, and a second electrode (a cathode or an anode) 209, and a TFT for driving the monitor element. Note that the gate wiring 206 is connected with a gate wiring provided in a pixel portion. A region 204 surrounded by dotted lines in FIG. 1 and FIG. 2 shows a region where a monitor element including an anode, an electroluminescent layer, and a cathode emits light. The gate wiring 205 of a TFT for driving the monitor element is overlapped with a first electrode 207 of the monitor element portion. Further, the TFT 211 is formed in the region surrounded by the control line 201 and the gate wiring 206, and the gate electrode 205 of the TFT 211 is formed over the same layer as a light shielding film 203, the light shielding film 203 is required to be formed so as not to overlap with the TFT 211. Therefore, it has been difficult to form the light shielding film 203 with an adequate size and shape to block the light. As a result, light generated from the monitor element is leaked from an aperture between the TFT 211 and the light shielding film 203. Furthermore, light is also leaked from a region which corresponds to an aperture between the current supply line 202 which connects the source region of a TFT for driving the monitor element and a constant current source, and the control line 201 which supplies electric potential to the gate wiring of the TFT for driving the monitor element. In FIG. 2, a reference numeral 210 is an interlayer insulating film, and 211 is an insulating film (referred to as a bank, a partition, a barrier, a mound, or the like).
This light leakage can be prevented by lowering the aperture ratio of the monitor element portion. However, the aperture ratio of the monitor element portion and that of a pixel portion are desirably at a comparable level in view of deterioration properties of a light-emitting element. Thus, it is not desirable to lower the aperture ratio of the monitor element portion for an original purpose to compensate the temperature of the light-emitting element of the pixel portion and deterioration thereof.
It is an object of the present invention to provide a display device in which a light shielding film without increasing the number of steps and causing high cost is formed.